It's common knowledge that programmers are lazy.
This shouldn't be considered as a negative characteristic, because in most cases it makes them more productive.
However, there are cases where this laziness makes programs/scripts execute way slower than they could. Let's take jQuery as an example. jQuery enables programmers to manipulate the DOM without much knowledge of what is happening on the background. This is not bad in most cases, because it makes the learning curve less steep, but in other cases degrades performance.
Element selection based on id (jQuery)
Element selection based on id (native)
Element selection based on id (native + jQuery)
As you might guess, the native method is the fastest, while jQuery was the slowest one.
The combination of the two methods (native + jQuery) is still twice as fast as only using jQuery.
See: http://jsperf.com/tschuermans-jquery-id-selector-vs-native-id-selector for a benchmark.
Element creation (jQuery example 1)
Element creation (jQuery example 2)
Element creation (jQuery example 3)
Element creation (Native)
Element creation (Native + jQuery)
Native methods beat jQuery again! The first three examples are similar in performance, while the native method is 3-4x faster. The combination of the two methods (native + jQuery) is still twice as fast as only using jQuery. See http://jsperf.com/tschuermans-element-creation for a benchmark.